Nitride semiconductor structure and semiconductor light emitting device including the same

ABSTRACT

A nitride semiconductor structure and a semiconductor light emitting device including the same are revealed. The nitride semiconductor structure mainly includes a stress control layer disposed between a light emitting layer and a p-type carrier blocking layer. The p-type carrier blocking layer is made from Al x Ga 1−x N (0&lt;x&lt;1) while the stress control layer is made from Al x In y Ga 1−x−y N (0&lt;x&lt;1, 0&lt;y&lt;1, 0&lt;x+y&lt;1). The light emitting layer has a multiple quantum well structure formed by a plurality of well layers and barrier layers stacked alternately. There is one well layer disposed between the two barrier layers. Thereby the stress control layer not only improves crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer and the light emitting layer but also reduces effects of compressive stress on the well layer caused by material differences.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor structure and a semiconductor light emitting device including the same, especially to a nitride semiconductor structure in which a stress control layer made from Al_(x)In_(y)Ga_(1−x−y)N is disposed between a light emitting layer and a p-type carrier blocking layer to improve crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer and the light emitting layer, increase the yield rate, and further reduce effects of compressive stress on quantum well layers. Thus electrons and holes are effectively confined in each quantum well layer and internal quantum efficiency is increased. Therefore the semiconductor light emitting device has a better light emitting efficiency.

2. Description of Related Art

In recent years, light emitting diodes (LED) have become more important in our daily lives due to their broad applications. LED is going to replace most of lighting devices available now and becoming a solid lighting element for the next generation. It's a trend to develop high energy saving, high efficiency and high power LED. Nitride LED has become one of the most popular optoelectronic semiconductor materials due to the advantages of compact volume, mercury-free, high efficiency and long service life. The wavelength of III-nitride covers almost covers the wavelength range of visible light so that it is a LED material with great potential.

Generally, for manufacturing nitride LED, firstly a buffer layer is formed on a substrate. Then an n-type semiconductor layer, a light emitting layer and a p-type semiconductor layer are formed over the buffer layer in turn by epitaxial growth. Next use photolithography and etching processes to remove a part of the p-type semiconductor layer and a part of the light emitting layer until a part of the n-type semiconductor layer is exposed. Later an n-type electrode and a p-type electrode are respectively formed on the exposed n-type semiconductor layer and the p-type semiconductor layer. A light emitting diode device is produced at last. The light emitting layer is in a multiple quantum well (MQW) structure formed by quantum well layers and quantum barrier layers disposed alternately. The band gap of the quantum well layer is lower than that of the quantum barrier layer, so that electrons and holes are confined in each quantum well layer of the MQW structure. Thus electrons and holes are respectively injected from the n-type semiconductor layer and the p-type semiconductor layer to be recombined with each other in the quantum well layers and photons are emitted.

However, the light efficiency of the LED can be affected by a plurality of factors such as current crowding, dislocation, etc. In theory, the light efficiency of LED is determined by external quantum efficiency, internal quantum efficiency and light-extraction efficiency. The internal quantum efficiency depends on material properties and quality. As to the light-extraction efficiency, it is defined as the ratio of the amount of light generated in the device and the amount of light escaping the device and radiated to the air. The light-extraction efficiency depends on the loss occurred while the light escaping the device. One of the main factors for the above loss is that the semiconductor material on the surface of the device has high refraction coefficient, so that total reflection occurs on surface of the material and photons are unable to be emitted. Once the light-extraction efficiency is improved, the external quantum efficiency of the semiconductor light emitting device is also increased. Thus various techniques for improving the internal quantum efficiency and the light-extraction efficiency have been developed in recent years. For example, the techniques include using indium tin oxide (ITO) as a current spreading layer, using the flip-flop, using patterned-sapphire substrate (PSS), using the current block layer (CBL), etc. Among the techniques used to improve the internal quantum efficiency, a method is to dispose a p-type carrier blocking layer (p-AlGaN) with high band gap between a multiple quantum well (MQW) structure and a p-type semiconductor layer. Thus more carriers are confined in the quantum well layers to increase electron-hole recombination rate and further improve light emitting efficiency. Therefore the brightness of LED is increased.

The MQW structure is generally formed by InGaN quantum well layers and GaN quantum barrier layers. Although the carriers can be effectively confined in the quantum well layers by using p-AlGaN as the p-type carrier blocking layer, there is high lattice mismatch between the p-AlGaN p-type carrier blocking layer and the GaN quantum barrier layer. Thus the InGaN quantum well layers are seriously affected by the compressive stress due to the lattice mismatch. The compressive stress changes band gap of each quantum well layer so that electrons and holes in the quantum well layers are separated from each other and the light emitting efficiency of the LED is reduced. Moreover, the compressive stress also degrades the adjacent GaN quantum barrier layers and interface properties among the InGaN quantum well layers so that carriers are lost at the interface and the light emitting efficiency of the LED is also affected.

Thus there is a room for improvement and a need to provide a novel nitride semiconductor structure and a semiconductor light emitting device including the same that overcome the above shortcomings.

SUMMARY OF THE INVENTION

Therefore it is a primary object of the present invention to provide a nitride semiconductor structure in which a stress control layer made from Al_(x)In_(y)Ga_(1−x−y)N is disposed between a light emitting layer and a p-type carrier blocking layer for improving crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer and the light emitting layer, increasing the yield rate, and reducing effects of compressive stress on well layers. Thus electrons and holes are effectively confined in each well layer and the internal quantum efficiency is increased. Therefore a semiconductor light emitting device having the nitride semiconductor structure provides a better light emitting efficiency.

It is another object of the present invention to provide a semiconductor light emitting device including the above nitride semiconductor structure.

In order to achieve above objects, a nitride semiconductor structure of the present invention includes a stress control layer disposed between a light emitting layer and a p-type carrier blocking layer. The light emitting layer has a multiple quantum well (MQW) structure formed by a plurality of well layers and barrier layers stacked alternately. One well layer is disposed between two barrier layers. The barrier layer is doped with an n-type dopant at a concentration ranging from 10¹⁶ to 10¹⁸ cm⁻³ so as to reduce carrier screening effect and increase carrier-confinement in the barrier layer. Moreover, the p-type carrier blocking layer is made from Al_(x)Ga_(1−x)N (0<x<1) while the stress control layer is made from Al_(x)In_(y)Ga_(1−x−y)N (0<x<1, 0<y<1, 0<x+y<1). Moreover, the amount of indium in the stress control layer is controlled to be equal to or smaller than that of the well layer of the MQW structure so as to form the stress control layer whose energy gap (band gap) is larger than that of the well layer. Thus carriers are confined in the well layers of the MQW structure for increasing the electron-hole recombination rate and improving the internal quantum efficiency.

Furthermore, the stress control layer is doped with a p-type dopant at a concentration smaller than 10¹⁹ cm⁻³ and an n-type dopant at a concentration smaller than 10¹⁹ cm⁻³. The thickness of the stress control layer is ranging from 2 nm to 15 nm. The optimal thickness of the stress control layer is smaller than the thickness of the well layer of the MQW structure. The thin stress control layer can prevent stress accumulation and mismatch dislocation.

In addition, a p-type semiconductor layer is disposed over the p-type carrier blocking layer and an n-type semiconductor layer is disposed between the light emitting layer and the substrate. An n-type carrier blocking layer is disposed between the light emitting layer and the n-type semiconductor layer. The p-type semiconductor layer is doped with the p-type dopant at a concentration higher than 5×10¹⁹ cm⁻³ while its thickness is smaller than 30 nm. The n- type carrier blocking layer is made from Al_(x)Ga_(1−x)N (0<x<1).

The MQW structure of the light emitting layer is formed by InGaN well layers and GaN barrier layers. The InGaN well layer has a lower band gap than that of the GaN barrier layer so that electrons and holes are more easily to be confined in the well layers and the electron-hole recombination rate is increased.

A super lattice layer is disposed between the light emitting layer and the n-type carrier blocking layer to reduce the lattice mismatch and the dislocation density between the light emitting layer and the n-type carrier blocking layer.

A semiconductor light emitting device including the above nitride semiconductor structure of the present invention includes an n-type electrode and a p-type electrode used together for providing electric power. The Al_(x)In_(y)Ga_(1−x−y)N stress control layer can not only improve crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer and the light emitting layer but also reduce effects of compressive stress on the InGaN well layer caused by material differences. Thus electrons and holes in the well layers are accumulated and confined therein more effectively. Therefore the internal quantum efficiency is increased.

Moreover, the reduction of the compressive stress also improves interface properties between the adjacent barrier layers and the well layers and reduces carrier loss at the interface. Thus the internal quantum efficiency is increased to make the semiconductor light emitting device have a better light emitting efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein

FIG. 1 is a schematic drawing showing a cross section of an embodiment of a nitride semiconductor structure according to the present invention;

FIG. 2 is a schematic drawing showing a cross section of an embodiment of a semiconductor light emitting device including a nitride semiconductor structure according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following embodiments, when it is mentioned that a layer of something (or membrane) or a structure is disposed over or under a substrate, another layer of something (or membrane), or another structure, that means the two structures, the layers of something (or membranes), the layer of something and the substrate, or the structure and the substrate can be directly or indirectly connected. The indirect connection means there is at least one intermediate layer disposed therebetween.

Referring to FIG. 1, a nitride semiconductor structure of the present invention mainly includes a light emitting layer 5, a p-type carrier blocking layer 7 and a stress control layer 6 disposed between the light emitting layer 5 and the p-type carrier blocking layer 7. The light emitting layer 5 is in a multiple quantum well (MQW) having a plurality of well layers 51 and barrier layers 52 interleaved with each other. One well layer 51 is interposed between the two barrier layers 52. The p-type carrier blocking layer 7 is made from Al_(x)Ga_(1−x)N (0<x<1) while the stress control layer 6 is made from Al_(x)In_(y)Ga_(1−x−y)N (0<x<1, 0<y<1, 0<x+y<1).

In the above embodiment, the barrier layer 52 is doped with an n-type dopant at a concentration ranging from 10¹⁶˜10¹⁸ cm⁻³. A p-type semiconductor layer 8 is disposed over the p-type carrier blocking layer 7 and is doped with a p-type dopant at a concentration higher than 5×10¹⁹ cm⁻³. The thickness of the p-type semiconductor layer 8 is smaller than 30 nm. An n-type semiconductor layer 3 is disposed between the light emitting layer 5 and a substrate 1. Moreover, in this embodiment, there is an n-type carrier blocking layer 4 disposed between the light emitting layer 5 and the n-type semiconductor layer 3. The n-type carrier blocking layer 4 is made from Al_(x)Ga_(1−x)N (0<x<1). A super lattice layer 9 is disposed between the light emitting layer 5 and the n-type carrier blocking layer 4 to reduce lattice mismatch and dislocation density between the light emitting layer 5 and the n-type carrier blocking layer 4.

In this embodiment, the stress control layer 6 is doped with a p-type dopant at a concentration smaller than 10¹⁹ cm⁻³ and an n-type dopant at a concentration smaller than 10¹⁹ cm⁻³. The preferred p-type dopant is magnesium while the optimal n-type dopant is silicon. The p-type dopant is used as a receptor to increase the effective hole concentration while the n-type dopant is a donor for improving crystallization of the gallium nitride (GaN) based semiconductor layers. By doping the p-type dopant and the n-type dopant at the same time, good electro-optical properties are produced. The thickness of the above stress control layer 6 is ranging from 2 nm to 15 nm. The preferred thickness of the stress control layer 6 is smaller than the thickness of the well layer 51 of the multiple quantum well (MQW) structure.

While in use, the n-type semiconductor layer 3 is made from Si-doped gallium nitride while materials for the p-type semiconductor layer 8 are Mg-doped gallium nitride. The preferred MQW structure of the light emitting layer 5 is composed of InGaN well layers 51 and GaN barrier layers 52. As to the stress control layer 6 made from Al_(x)In_(y)Ga_(1−x−y)N, it is located between the p-type carrier blocking layer 7 and the light emitting layer 5.

By control of the amount of indium in the stress control layer 6 to make the amount of indium n the stress control layer 6 become equal or lower than the well layers 51 of the MQW structure, the stress control layer 6 whose energy gap is larger than the well layer 51 is formed. Thus carriers are confined in the well layers 51 of the MQW structure to increase electron-hole recombination rate and further improve internal quantum efficiency. Therefore the light emitting efficiency of the semiconductor light emitting device is significantly improved.

In addition, the Al_(x),In_(y)Ga_(1−x−y)N stress control layer 6 of the present invention is not only used as a buffer layer between the p-type carrier blocking layer 7 and the light emitting layer 5, it's also able to improve crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer 7 and the light emitting layer 5 as well as reduce effects of compressive stress on the well layer 51 because that the band gap of InGaN containing indium is smaller than that of GaN while the band gap of AlGaN containing aluminum is larger than that of GaN. Thus electron and hole accumulation occurs in the well layer 51. Both electrons and holes are confined in adjacent well layer 51 so as to increase the internal quantum efficiency. Furthermore, the reduction of the compressive stress also enhances interface properties between the adjacent GaN barrier layer 52 and the InGaN well layer 51 and improves carrier loss at the interface. Thus the internal quantum efficiency is increased.

The nitride semiconductor structure is applied to semiconductor light emitting devices. Referring to FIG. 2, a cross section of a semiconductor light emitting device including the nitride semiconductor structure of an embodiment according to the present invention is revealed. The semiconductor light emitting device includes at least: a substrate 1, an n-type semiconductor layer 3 disposed over the substrate 1 and made from Si-doped GaN, a light emitting layer 5 disposed over the n-type semiconductor layer 3 and having a multiple quantum well structure, a stress control layer 6 disposed over the light emitting layer 5, a p-type carrier blocking layer 7 disposed over the stress control layer 6 and made from Al_(x)Ga_(1−x)N (0<x<1), a p-type semiconductor layer 8 disposed over the p-type carrier blocking layer 7 and made from Mg-doped GaN, an n-type electrode 31 disposed on and in ohmic contact with the n-type semiconductor layer 3, and a p-type electrode 81 disposed at and in ohmic contact with the p-type semiconductor layer 8.

The multiple quantum well structure of the light emitting layer is formed by a plurality of well layers 51 and a plurality of barrier layers 52 stacked alternately. Each well layer 51 is disposed between two barrier layers 52. The well layer 51 and the barrier layer 52 are respectively made from InGaN and GaN. Thereby electrons and holes are more easily to be confined in the well layer 51 so that the electron-hole recombination rate increased and the internal quantum efficiency is improved.

The stress control layer 6 is made from Al_(x)In_(y)Ga_(1−x−y)N while x and y satisfy following conditions: 0<x<1, 0<y<1, and 0<x+y<1. In this embodiment, the stress control layer 6 is doped with a p-type dopant (Mg is preferred) at a concentration smaller than 10¹⁹ cm⁻³ and an n-type dopant (Si is preferred) at a concentration smaller than 10¹⁹ cm⁻³. The thickness of the stress control layer 6 is ranging from 2 nm to 15 nm and this thickness is smaller than the thickness of the well layer 51. Moreover, aluminum ions in the p-type carrier blocking layer 7 are going to diffuse into the stress control layer 6 so that the amount of indium in the stress control layer 6 is equal to or smaller than the well layer 51 of the MQW structure. Thus the stress control layer 6 whose band gap is larger than that of the well layer 51 is formed. Therefore carriers are restricted in the well layers 51 of the MQW structure to increase the electron-hole recombination rate and improve the internal quantum efficiency.

The n-type electrode 31 and the p-type electrode 81 are used together to provide electric power and are made from (but not limited to) the following materials: titanium, aluminum, gold, chromium, nickel, platinum, and their alloys. The manufacturing processes are well-known to people skilled in the art.

Moreover, the semiconductor light emitting device further includes an n-type carrier blocking layer 4 and a buffer layer 2. The n-type carrier blocking layer 4 is disposed between the light emitting layer 5 and the n-type semiconductor layer 3 while the buffer layer 2 is disposed between the n-type semiconductor layer 3 and the substrate 1. The n-type carrier blocking layer 4 is made from material Al_(x)Ga_(1−x)N (0<x<1) so that carriers are confined in the well layers 51. Thus the electron-hole recombination rate is increased, the light emitting efficiency is improved, and the brightness of the semiconductor light emitting device is further enhanced. The buffer layer 2 is made from Al_(x)Ga_(1−x)N (0<x<1) and is used for solving the dislocation problem caused by lattice mismatch between the substrate 1 and the n-type semiconductor layer 3.

In summary, according to the above embodiments, the Al_(x)In_(y)Ga_(1−x−y)N stress control layer 6 of the semiconductor light emitting device not only solves the problem of crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer 7 and the light emitting layer 5 for increasing the yield rate. It also reduces effects of compressive stress on the InGaN well layer 51 caused by material differences. Thus electrons and holes accumulate and confined more effectively in the well layer 51 so as to increase the internal quantum efficiency. Moreover, the reduction of the compressive stress also enhances interface properties between the adjacent barrier layers 52 and the well layers 51 and improves carrier loss at the interface. Therefore the internal quantum efficiency is increased and the semiconductor light emitting device gets a better light emitting efficiency.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A semiconductor light emitting device comprising: a substrate; an n-type semiconductor layer disposed over the substrate; a light emitting layer disposed over the n-type semiconductor layer and having a multiple quantum well (MQW) structure; the MQW structure having a plurality of well layers and barrier layers stacked alternately; one of the well layers being disposed between two of the barrier layers; a stress control layer disposed over the light emitting layer and made from Al_(x)In_(y)Ga_(1−x−y)N while x and y satisfy the conditions: 0<x<1 , 0<y<1, and 0<x+y<1; a p-type carrier blocking layer disposed over the stress control layer and made from Al_(x)Ga_(1−x)N, wherein 0<x<1; a p-type semiconductor layer disposed over the p-type carrier blocking layer; a n-type electrode disposed on and in ohmic contact with the n-type semiconductor layer; and a p-type electrode disposed at and in ohmic contact with the p-type semiconductor layer.
 2. The semiconductor light emitting device as claimed in claim 1, wherein the stress control layer is doped with a p-type dopant at a concentration smaller than 10¹⁹cm⁻³.
 3. The semiconductor light emitting device as claimed in claim 1, wherein the stress control layer is doped with an n-type dopant at a concentration smaller than 10¹⁹ cm⁻³.
 4. The semiconductor light emitting device as claimed in claim 1, wherein the amount of indium in the stress control layer is equal to or smaller than the amount of indium in the well layer of the MQW structure.
 5. The semiconductor light emitting device as claimed in claim 1, wherein a thickness of the stress control layer is ranging from 2 nm to 15 nm.
 6. The semiconductor light emitting device as claimed in claim 1, wherein a thickness of the stress control layer is smaller than a thickness of the well layer of the MQW structure.
 7. The semiconductor light emitting device as claimed in claim 1, wherein the barrier layer is doped with an n-type dopant at a concentration ranging from 10¹⁶ to 10¹⁸ cm⁻³.
 8. The semiconductor light emitting device as claimed in claim 1, wherein the p-type semiconductor layer is doped with a p-type dopant at a concentration higher than 5×10¹⁹ cm⁻³ and a thickness of the p-type semiconductor layer is smaller than 30 nm. 